‘Partial herd immunity’ emerging, but some challenge the value of that status
It was nearly a year ago when Gov. Gavin Newsom, not long into a statewide lockdown, referenced the elephant in the COVID-19 room: herd immunity.
The concept was frightening to many; with no vaccine in sight, they feared the only way it could be attained was for the virus to have infected enough people to have run out of hosts. Estimates of the looming toll at the time were chilling: as many as 840,000 Californians dead and 1.4 million seriously ill.
National, state and local officials, meanwhile, worked to fend off infection with face masks, social distancing and scaled-back public engagement. That would change life as we knew it — “at least until we have herd immunity,” the governor said.
A year later, with the COVID-19 death toll reaching the 500,000 mark and despite a sluggish vaccine rollout, infection and hospitalization rates are falling dramatically, empowering some officials to declare that “partial” herd immunity has been achieved. Some say that as many as one-third of Americans might have already had the virus and call that the indicator of a waning pandemic.
Others argue that “partial” herd immunity means very little. Only more complete exposure — upwards of 80-90%, some say — must kick in before society can exhale in the battle against the virus, they contend.
The official numbers that the public watches on state and local dashboards are based on confirmed cases. But many cases still go unrecognized or unreported. Epidemiologists work to calculate what they believe to be true infection levels, factoring in the rate at which the virus is being transmitted and how many people already have recovered.
In Los Angeles County, for instance, public health officials estimate that as many one-third of its 10 million residents have had coronavirus.
Jeffrey Shaman, a lead researcher and infectious disease specialist at Columbia University, estimates that 120 million Americans — five times the official count — have been infected. That’s just over one-third of the U.S. population.
“I do think that may be contributing to the decline in cases,” said Dr. Paul Simon, L.A. County’s chief science officer. “While overall we think 30% to 40% of the population has had past infection and has had some level of immunity, that’s not evenly distributed across the county. There are some places where many more people have had the virus and others where they have not, and there are social networks where many have had infection, so within those networks we think there may be less opportunity and less chance for transmission.”
In the Inland Empire, a similar trend has emerged.
“We agree that we are beginning to see what can be referred to as a partial herd immunity effect,” said Dr. Corwin Porter, San Bernardino County’s public health director. “Our county had significant viral transmission in the last several months leading to many residents having natural immunity to the virus.”
Nonetheless, that isn’t enough to trigger any kind of a “return to normal,” said Dr. Clayton Chau, director of Orange County’s Health Care Agency.
“A large number of our communities must have had the vaccine in order for us to safely go without (masks),” Chau said. A fully vaccinated person still could develop an asymptomatic case of COVID-19 and spread it to others who haven’t received shots, he added.
The number of people who have been vaccinated in Orange County is still “very small,” Chau said. “In order for Orange County to go back to near-normal, we have to reach herd immunity,” he said.
It’s a refrain heard across the region.
“Regarding herd immunity, vaccination is the number one thing we need to focus on,” said Dr. Brandon Brown, associate professor in public health at UC Riverside’s School of Medicine. “While a high percentage of people in L.A. County and Riverside may have already been exposed to the virus, the variants are a game changer, which is why we have to continue practicing the public health measures that we know protect us against the virus (masks, physical distancing, handwashing, vaccination).”
“I do think we have fewer people who are likely to get infected,” said L.A. County Public Health chief Barbara Ferrer. “People we know were infected have some protections, as do people who are vaccinated. And that does help us in slowing the spread. But I do think the most important reason these transmissions are coming down is because, in fact, people are back in the game and doing their very best to help each other, and we’ll need to keep doing that for the next few months in order to stay on our recovering journey.”
No one knows absolutely when the “herd immunity moment” will arrive.
“We don’t quite know yet what level of vaccination or protection from infection would be required to get to herd immunity,” Simon said. “Experts have speculated anywhere from 50% to 60%, which many think is too low, to higher estimates of 80% to 90%. But I think we will know over time as we see the number of new cases drop dramatically. That will be a way to know if we are seeing herd immunity. And then we can relax some of the public health measures.”
Officials also caution that while about one-third of people are estimated to have had the virus in some form, that still leaves many who have not.
“About 60% to 70% of the population continue to be susceptible, so I think we need to continue to really be vigilant by adhering to the social guidelines,” Simon said.
It’s too early to rest easy, said epidemiologist Dr. Michael Osterholm, who told C-SPAN last week that we should expect “more curve balls to get thrown at us.”
Osterholm warned against forgetting about past surges, largely fueled by public gatherings.
New, faster-spreading strains could scramble the recovery strategy, too. “If some of the variants develop a foothold, then the level of vaccination coverage will need to be even higher,” Simon said.
Osterholm and Simon also noted that the declining baseline for case numbers remains very high, a point echoed over the weekend by Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and chief White House medical adviser.
“I’m not sure this is herd immunity,” he told “Meet the Press.” While the steep drop in cases is highly encouraging, he said, it’s easing downward from a mammoth national caseload that overwhelmed hospital staffs and overloaded morgues and mortuaries.
Ultimately, officials say, they’ll know more true mass immunity when they see it.
“If we get to the point where there isn’t community-based transmission, that will be an indication that some combination of people being careful and herd immunity is preventing the virus from being spread easily,” said Dr. Roger Lewis, director of COVID-19 Demand Modeling for L.A. County’s Department of Health Services.
Either way, fear is easing for many people, said Dr. Thomas W. LaGrelius, a Torrance geriatrician. Early on, people could only hearken back to the 1918-19 Spanish flu pandemic, which claimed 675,000 U.S. lives and 50 million globally. “It was assumed that a vaccine might take several years to develop,” he said. It arrived in months.
But now LaGrelius and others are optimistic, projecting that within three to four months, a combination of vaccine and natural spread may well result in herd immunity.
“We’re getting up there,” he said.